Thermochromic material

ABSTRACT

Thermochromic material comprising a thermochromic component and a binder, wherein the thermochromic component is crystal phases based on oxides of heavy metals of I, II, III, IV, V, VI, VII, VIII groups of the Periodic System selected from the group consisting of compounds of the following general formulae:  
     (i) (Bi 2 O 3 ) 1-z (M x O y ) z  where z=0-0.5, wherein M is selected from the group consisting of heavy, alkali, alkaline earth metals and mixtures thereof;  
     (ii) (M x O y ) m (Bi 2 O 3 ) n Nb(Ta) 2 O 5 , where m=0-1, n=1-2, wherein M is selected from the group consisting of heavy, alkali, alkaline earth metals and mixtures thereof;  
     (iii) (M x O y ) m  (Bi 2 O 3 ) n Mo(W)O 3  where m=0-1, n=0-12, wherein M is selected from the group consisting of alkali, alkaline earth, heavy metals and mixtures thereof.  
     (iv) (M x O y ) m (Me x O y ) n Mo(W,Cr)O 3  where m=0-1, n=0-1, wherein M is selected from the group consisting of alkali/alkaline earth, heavy metals and mixtures thereof, and Me is a heavy metal;  
     (v) (M x O y ) m (Me x O y ) n Nb(Ta) 2 O 5 , where m=0-1, n=0-1, wherein M is selected from the group consisting of alkali, alkaline earth, heavy metals and mixtures thereof, and Me is selected from the group consisting of Cu(II), Mn(II), Mn(III), Co(II), Ni(II), Cr(III),  
     the ratio in terms of weight percentage of thermochromic component versus binder being from 2:98 to 98:2, and use of compounds (i) to (v) as thermochromic components are disclosed.

[0001] This invention relates to thermochromic materials, in particular,thermochromic coatings, change of color in which may be used fortemperature indication. One of the fields for thermochromic materialsapplication is household appliances with coating of thermochromicmaterials. The use of thermochromic materials permits to simply andefficiently warn a customer about danger of touching a certain portionof an article.

[0002] Requirements to thermochromic coatings of household appliancesare as follows:

[0003] The coating should signal about temperature of the surface withinthe range of from 100 to 400° C., which may cause a burn. Highertemperature, for instance, on cooking top of stove surface may be felton account of heat radiation or visible red color of the surface.

[0004] Change of color should be reversible in the heatingcycles—cooling without effects of aging and solarization(discoloration).

[0005] The coating should be stable up to maximum operation temperature(700° C. for cooking tops, 400° for other devices).

[0006] The coating should meet sanitary engineering requirements andnorms in respect of toxicity.

[0007] With such requirements the art-known thermochromic materialsbased on organic compounds and liquid crystals cannot be used, sincethey are not stable at high (up to 700° C.) temperatures, therefore theyare not considered in the background of the invention.

[0008] Known in the art are thermochromic materials on the basis ofcadmium and mercury sulfides and selenides as thermochromic componentsand lead-silicate enamels (U.S. Pat. No. 5,772,328 and No. 5,499,597) orborosilicate glass (U.S. Pat. No. 4,983,810) as a binder. The art-knownthermochromic materials permit to obtain coatings stable to temperaturesup to 700° C. However, in accordance with the current norms the coatingson their base cannot be used for applying onto the surfaces of householdappliances because of high toxicity rates of cadmium and mercury.

[0009] The solution disclosed in U.S. Pat. No. 4,983,810 is believed tobe the closest to the claimed one, in compliance with this solution thethermochromic materials comprises:

[0010] as thermochromic component the compounds of formulaCdS_(1-x)Se_(x), wherein x=0-0.8, or formula Zn_(1-y)Mn_(y)O whereiny=0.05-0.15;

[0011] as binder glasses or glass ceramic, in particular, borosilicateglass;

[0012] as non-thermochromic or low-thermochromic compound the color ofwhich is close to that of thermochromic component, for instance,Pb₃(SbO₄)₂ or ZrO₂ alloyed with praseodymium. This component is used asthe inner standard of color transition in the thermochromic componentupon heating.

[0013] It was already mentioned above that cadmium compositions aretoxic and cannot meet the requirements of sanitary engineering norms inrespect of toxicity valid for household appliances. Regarding thecompound Zn_(1-y)Mn_(y)O according to the Specification it is stableupon heating to 400° C. only and has no full reversibility of colortransition due to oxidation.

[0014] The purpose of this invention consists in creation of athermochromic material, non toxic, the color of which is reversible attemperature change from room temperature to 400° C., and, of which thecolor transition should permit to reflect temperature changes of lessthan by 200° C., and which is stable when heated to 700° C.

[0015] This purpose is attained by that a thermochromic material isdisclosed which contains as a thermochromic component based on crystalof heavy metals of I, II, III, IV, V, VI, VII, VIII groups of thePeriodic System, and as the binder—mixtures or pure components on thebasis of silicates, borates, phosphates of alkali or alkaline earthmetals, the weight ratio of the thermochromic component to binder beingfrom 2:98 to 98:2.

[0016] For intensification of thermochromic features the thermochromicmaterials may additionally comprise a thermostable non-thermochromic orlow-thermochromic component, of which the area of diffusion reflectionmaximum lies in the same or is close to the spectral range where thetemperature-related change of diffusion reflection spectrum of the basicthermochromic component lies.

[0017] A distinctive feature of the claimed invention is thethermochromic component which is selected from the group of thefollowing compounds:

[0018] (i) Based on bismuth oxide compound of the general formula(Bi₂O₃)_(1-z)(M_(x)O_(y))_(z) at z=0-0.5, wherein M is selected from thegroup consisting of heavy, alkali, alkaline earth metals and mixturesthereof. For instance, M is Zr (IV), Hf(IV), Sn(II), Sn(IV), Nb(V),Ta(V), Mo(VI), W(VI), Cr(III), Cr(VI), Mn(II), Fe(III), Co(II), Ni(II),Pb(II), Ca(II), Sr(II), Ba(II), Li, Na, K, Rb, Cs.

[0019] (ii) Niobates and tantalates of general formula(M_(x)O_(y))_(m)(Bi₂O₃)_(n)Nb(Ta)₂O₅, with m=0-1, n=1-2, wherein M isselected from the group consisting of heavy, alkali, alkaline earthmetals and mixtures thereof. For instance, M is Na, K, Rb, Cs, Mg, Ca,Sr, Ba, Pb(II), Co (II), Ni(II), Cr(III), Cu(II), Cu(I).

[0020] (iii) Molibdates and tungstates of general formula(M_(x)O_(Y))_(m)(Bi₂O₃)_(n)Mo(W)O₃ at m=0-1, n=0-12, wherein M isselected from the group consisting of alkali, alkaline earth, heavymetals and mixtures thereof. For instance, M is Na, K, Rb, Cs, Mg, Ca,Sr, Ba, Sn, Ti, Zr, Pb(II), Mn(II), Mn(III), Co(II), Ni (II), Cr(III),Cu(II).

[0021] (iv) Chromates, molibdates, tungstates of general formula(M_(x)O_(y))_(m)(Me_(x)C_(y))_(n)Mo(W,Cr)O₃ at m=0-1, n=0-1, wherein Mis selected from the group consisting of alkali, alkaline earth, heavymetals and mixtures thereof, Me is a heavy metal. For instance, M is Na,K, Pb, Cs, Mg, Ca, Sr, Ba, Sn, Ti, Zr, Pb(II), and examples of Me areCu(II), Mn(II), Mn(III), Co(II), Ni(II), Cr(III).

[0022] (v) Niobates and tantalates of general formula(M_(x)O_(y))_(m)(Me_(x)O_(y))_(n)Nb(Ta)₂O₅, at m =0-1, n=0-1, wherein Mis selected from the group consisting of alkali, alkaline earth, heavymetals and mixtures thereof, and Me is selected from the groupconsisting of Cu(II), Mn(II), Mn(III), Co(II), Ni(II), Cr(III). Forinstance, M is Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Sn, Ti, Zr, Pb(II).

[0023] An example of stable non-thermochromic or low thermochromicpigment is blue cobalt CoAl₂O₄ or CoWO₄ or Co_(1-x)Zn_(x)WO₄ or CoMoO₄for the thermochromic compound Bi₂O₃ or the compounds are selected from(ii) and (iii). Another example is the mixture of (Bi₂O₃)_(x)(CrO₃)_(x)as a thermochromic component and Cr₂O₃ as thermostable pigment. Theratio of the quantity of thermochromic compound to thermochromic pigmentis in the range of from 50:1 to 1:30.

[0024] Traditionally, the phenomenon of thermochromism is connected withphase transition in solid sate (polymorphic transformation). Typicalrepresentatives of solid thermochromic compounds of this type are someof iodomercurates, iodides of tallium, mercury, silver, which have clearand reversible color change in the point of phase transition. (J. H.Day. Thermochromism of Inorganic Compounds. Chem. Rev., 68 (1968), 669;K. Sone, Y. Fukuda. Inorganic Thermochromism. Springer-Verlag, Berline.a., 1987). These compounds have high contrast of color changes withtemperature, but they are stable only at low temperatures. For mostthermochromic compounds, such as Ag₂HgI₄ maximum allowable temperaturedoes not exceed 200° C. (D. Negoin, T. Rosu. Electric, thermal andthermochromic properties of M_(x)HgI₄-type compounds. Rev. Chem., 45(1994), 201). It is not sufficient for application of thermochromiccoatings in such articles as kitchen ovens, temperature of thecooking-top in which may reach 700° C.

[0025] Known in the art are heat resistant thermochromic oxides on thebasis of the compounds based on the structures of aluminium-chromiumsubstitution, for instance, rubies and spinels (C. P. Poolle. Theoptical spectra and color of Chromium containing solids. J. Phys. Chem.Dolids, 25 (1964), 1169).

[0026] Thermochromism of art-known compositions of rubies d-elements(Al_(2-x)Cr_(x)O₃) and spinels (MgAl_(2-x)Cr_(x)O₄), as well as of theclaimed ones, is stipulated not by the phase transition with thetemperature changes, but with the change in ligands field force. Colorchange takes place with chromium concentration increase on account ofaluminium atoms with chromium atoms substitution, which is accompaniedby lattice deformation due to greater radius of chromium atoms againstaluminium atoms. Hereupon, the phenomenon of such thermochromism isknown for chromium only.

[0027] If chromium concentration in these compounds is not high, theyhave pink color. At high chromium concentrations the color of thesecompounds is green. Pink crystals have thermochromism: upon heatingtheir color gradually changes from pink at low temperatures to green athigh temperatures. However, this change takes place very slowly withinwide range of temperatures from 200 to 900° C. Within the range oftemperatures from room temperature to about 400° C., which is the mostsubstantial for warning a customer, change of color in rubies andspinels is not sufficient for using them as thermochromic components ofthe coatings.

[0028] Thermochromic component disclosed in this work have the featureof reversibly changing their color within wide range of temperatures andin such way that temperature change by 100 becomes visible, hereupon,the coatings based on them have thermal stability of up to 700° C.

[0029] Here for the first time not only the above features have beenfound, but for the first time such solid compounds have been found whichmanifest strong contrast color change with the temperature and arestable in the air at the temperature up to 700° C.

[0030] The above compounds were obtained by a standard method ofpressing the mixture of initial oxides followed by heat treatment at700-1100° C. for 4-100 hours, depending on the composition. Phasecomposition of the resultant compounds was defined by x-ray phaseanalysis on difractometer DRON-2, chemical composition was controlled byx-ray spectral analysis.

[0031] Thermochromic features of the resultant thermochromic compoundswere tested, taking spectra of powders diffusion reflection with respectto temperature, which was changed within the range from room to 400° C.For measurement purposes spectrophotometer SF-26 was used provided witha special cell with a heater, arranged in spectrophotometer in place ofstandard holder for powder samples. Shift of the diffusion absorptionband or change in spectrum shape indicated thermochromic effect.

[0032] Coating thermochromic properties may be characterized by twomethods depending on the type of diffusion spectrum and its behaviorupon temperature variations of the sample:

[0033] 1. Speed of color change with temperature. Coating color ischaracterized by a point with coordinates (x, y) on the color graph(color triangle), which are calculated from the reflection spectrum (M.M. Gurevich, E. F. Itsko, M. M. Seredenko. Optic properties ofvarnish-paint coatings. “Khimiya”, L. 1984). Thermochromism rate may becharacterized with a velocity of this point along the color graph withtemperature, i.e. the value${TX} = {\frac{\partial\sqrt{x_{2} + y_{2}}}{\partial T}.}$

[0034] An ordinary specialist is capable of distinguishing up to tenthousand colors if he compares two colors between them. It means, ifTX≈2×10⁻⁴, the operator can see the difference in temperatures betweencool and hot surfaces of about 100° C. For cadmium sulfide appliedaccording to the above mentioned USA patents this value is about 3×10⁻⁴.The advantage of estimating thermochromic features by this methodconsists in its absolute nature: the basis for comparing differentcoatings is their color. However, this method is labor consuming, sinceits practicing requires integration of sophisticated functions along thewhole spectrum. Moreover, an operator eye feels color changesdifferently in different ranges of the spectrum: smaller TX values areseen in the range of blue colors, bigger—in red.

[0035] 2. In many cases changes of reflection spectrum take placegradually as the edge of absorption shifts upon heating, in most casestowards the long wave area of the spectrum. Such behavior of thereflection spectrum is specific for semi-conductors, for instance, forcadmium, zinc and mercury sulfides and selenides used as thermochromiccomponent (see the above mentioned USA patents) . In the temperaturerange of from room and above position of the absorption edge (or thesame, edges of diffuse reflectance) is linear dependent on thetemperature, if expressed in energy units:

E(T)=E(T ₀)−β(T−T ₀),

[0036] wherein β is the temperature coefficient characterizingabsorption edge shift with temperature. Thermochromic properties of thecompound are expressed the stronger the greater is β value. For cadmiumsulfide, for instance, this value is:

β=7×10⁻⁴ eV/K.

[0037] After estimation of thermochromic properties of the compoundstheir powders were mixed with a binder selected from silicates,phosphates, borates and mixtures thereof, the total ratio ofthermochromic component to binder being from 2:98 to 98:2, doped withwater and the resultant composition was applied onto the surface ofglass, metal, or glass-ceramic. For improving cohesion of the coatingwith the glass-ceramic, glass or ceramic the latter was roughed byabrasive powder treatment or by means of groove cutting with the help ofa disc. Plate with the coating applied thereon was placed into stove andheated to a temperature of from 500 to 970° C. for 10 minutes to 1 hourdepending on the compounds composition. Thereafter, spectra of coatingdiffuse reflectance depending on temperature were tested.

[0038] Results of the spectral assays indicate that the claimed metaloxides and the coatings on the basis thereof have thermochromicproperties, expressed in reversible, strong and contrast color changeupon change of the temperature from room temperature to 400° C., stayingstable upon heating up to 700° C., hereupon, temperature change by 100°C. becomes visible.

[0039] The essence of this invention is better disclosed in the Examplesbelow, which do not restrict the scope of rights and are of mereillustrative nature.

EXAMPLE 1

[0040] This Example illustrates in detail the process for preparingthermochromic coating and the features thereof. Bismuth, lead, tantalumoxide powders were mixed at molar ratio of 1:1:1, total charge massbeing 20 grams. The mixture was pressed into a tablet with the help of ahydraulic press at the pressure of about 1000 kg/cm². The tablet wasplaced into the oven the temperature of which was being increasedgradually from room temperature to 800° C. within five hours. At thistemperature the tablet was left for 100 hours. After cooling of thestove the tablet was again grounded into powder, mixed, pressed andagain kept for 100 hours at the temperature of 800° C. X-ray phaseanalysis supports homogeneous nature of the resultant crystallinecompound PbO.Bi₂O₃.Ta₂0₅. This compound is thermochromic: upon heatingfrom 20 to 400° C. its color changed from light yellow to orange. Powderof this compound was mixed with glass frit of the following composition:78Ba(PO₃)₂.22Pb(PO₃)₂, mass ratio being 90:10, then water was added tothe mixture and the resultant composition was applied onto the surfaceof a glass-ceramic plate. The glass-ceramic plate with the coatingapplied was placed into the oven, the temperature of which was graduallyincreased to 500° C., and at this temperature the plate was kept for 1hour. The coating so obtained had light yellow color. FIG. 1 disclosesreflectance spectra, which indicate gradual shift of the absorption edgetowards the long wave area of the spectrum upon temperature increase,i.e. the resultant coating has thermochromic properties. As acharacteristic of thermochromic properties of the coating it is possibleto consider the dependence of the edge position on the temperature, forinstance, at the level of reflectance coefficient of 0.5. Thisdependence is disclosed in FIG. 2. It is evident that position of theclaimed coating absorption edge is in linear dependence on thetemperature, as it is in semiconducting compounds. Temperaturecoefficient of the edge shift coefficient of the considered coating is:

β=(7.4±0.3)×10⁻⁴ eV/K.

[0041] Thermochromism of the coating may be characterized also by thespeed of color change TX=2.3 ×10⁻⁴. These values are comparable with thethermochromism value, which may be attained of the coatings containingcadmium sulfide.

EXAMPLE 2

[0042] Similar to Example 1, the compound of the following composition:PbO.Bi₂O₃.2Ta₂O₅. The resultant compound was mixed with aluminophosphatebinder having approximate final formula Al₂O₃.3P₂O₅. Preparation and useof this binder was made in compliance with literature recommendations(M. M. Sychev. Non-organic glues, L. Khymia, 1986) . Mass ratio of thebinder (recalculated to solid matter) and thermochromic component in themixture used for coating application was equal to 1:9. The mixture wasapplied onto the glass-ceramic surface prepared similar to the methoddisclosed in Example 1. The coating was dried by heating to 450° C. andmaintained at this temperature for 1 hour. Properties of the coatingwere examined by the method similar to one presented in Example 1.According to measurements, the value of temperature coefficient ofabsorption edge shift for this coating is:

β=(6.7±0.4)×10⁻⁴ eV/K.

EXAMPLE 3

[0043] All steps were accomplished similar to Examples 1 and 2, butcomposition of the thermochromic component was PbO.Bi₂O₃.4Ta₂O₅, and asthe binder magnesium phosphate binder was used which had approximatecomposition of 2MgO.P₂O₅, the ratio being 70:30. The coating was heatedat 450° C. for 1 hour. The value of temperature coefficient ofabsorption edge shift for this coating was:

β=(8.2±0.3)×10⁻⁴ eV/K.

EXAMPLE 4

[0044] This Example illustrates preparation of thermochromic coating onthe basis of a compound containing bismuth oxide and having maximumthermochromism value. Using the same synthesis conditions as above inExamples 1 and 2 thermochromic component was obtained with thecomposition 7Bi₂O₃.Nb₂O₅. The coating was applied as in Example 2. Itsreflectance spectrum in the temperature range of from room temperatureto 400° C. is given in FIG. 3, and temperature reliance of theabsorption edge position—in FIG. 4. It is evident that for thisparticular coating thermochromism value is:

β=12.3 ×10⁻⁴ eV/K.

[0045] It corresponds to color change upon heating from light yellow atroom temperature to dark orange at 400° C. TABLE 1 Thermochromicproperties of coatings making use of bismuth oxide compound as athermochromic component COMPOSITION β × 10⁴, ev/K Na₂O Bi₂O₃ Ta₂O₅ 5.0BaO Bi₂O₃ Ta₂O₅ 6.6 PbO Bi₂O₃ Ta₂O₅ + CuO 5.6 PbO Bi₂O₃ Ta₂O₅ + NiO 5.5(BaNb₂O₆)_(0.6)(Bi_(2/3)Nb₂O₆)_(0.4) 2.5 CaO Bi₂O₃ Nb₂O₅ 5.5 PbO Bi₂O₃Nb₂O₅:NiO 5.2 Bi₂O₃ 11.1 8Bi₂O₃0.5Cr₂O₃ 7.8 7Bi₂O₃WO₃ 12.0 15Bi₂O₃Li₂O11.3 Bi₂O₃4Ta₂O₅ 3.1 CaO3.3Bi₂O₃ 9.4 7CaO3Bi₂O₃ 7.5 7Bi₂O₃CrO₃ 9.14Bi₂O₃CrO₃ 8.0 K₂O Bi₂O₃ Ta₂O₅ 6.8 PbO Bi₂O₃ Ta₂O₅ 7.4 PbO Bi₂O₃ Ta₂O₅ +Fe₂O₃ 5.3 PbO(Bi₂O₃)_(1/3)Nb₂O₆ 4.0 Ba₂BiNbO₆ 6.6 PbO Bi₂O₃ Nb₂O₅ 6.50.98Bi₂O₃0.02Cr₂O₃ 7.6 Bi₂O₃WO₃MoO₃ 7.1 Bi₂O₃2WO₃ 9.2 Bi₂O₃MoO₃ 7.815Bi₂O₃Na₂O 11.4 Bi₂O₃3Ta₂O₅ 5.4 7CaO5Bi₂O₃ 9.1 3Bi₂O₃CrO₃ 6.98Bi₂O₃CrO₃ 9.4 2Bi₂O₃3SnO₂ 4.1 CaO Bi₂O₃ Ta₂O₅ 6.2 PbO Bi₂O₃ Ta₂O₅ + CoO3.2 PbO Bi₂O₃ Ta₂O₅ + Nb₂O₃ 6.4 (Bi₂O₃)_(1/3)Nb₂O₅ 5.5 ZnO Bi₂O₃ Nb₂O₅5.5 (ZnO)_(0.9)(NiO)_(0.1)Bi₂O₃Nb₂O₅ 5.2 0.94Bi₂O₃0.06Cr₂O₃ 7.8Bi₂O₃3WO₃ 9.0 Bi₂O₃WO₃ 8.7 7Bi₂O₃MoO₃ 11.4 2Bi₂O₃3Ta₂O₅ 8.3 SrOBi₂O₃ 7.25CaO7Bi₂O₃ 8.2 5Bi₂O₃CrO₃ 8.5 3Bi₂O₃WO₃ 8.6 2Bi₂O₃3ZrO₂ 7.1

[0046] Table 1 summarizes temperature coefficient values of theabsorption layer shift for the coatings created on the basis of other 48compounds comprising in its composition bismuth oxide. All of them arethermochromic, and their thermochromism relies upon the absorption edgeshift to long-wave area of the spectrum upon heating.

EXAMPLE 5

[0047] This Example illustrates application of chromates as a componentof thermochromic coatings. Pouring together equivalent amounts ofpotassium chromate and Barium chloride resulted in Barium chromateresidue of bright yellow color. The residue was filtered and mixed withborosilicate frit of glass enamel TIT24, the ratio being 30:70.

[0048] After adding water the resultant mixture was deposited onto thesurface of a glass plate used as the initial material for obtainingglass-ceramic tile. The glass plate was placed into the ceramming oven,wherein the maximum temperature was 970° C., and maintenance at thistemperature was 30 minutes. Coating reflectance spectra are given inFIG. 5, and temperature reliance of the absorption edge—in FIG. 6. Theresultant coating has yellow color with greenish tint at roomtemperature. Coating color changes to bright orange upon temperatureincrease from room temperature to 400° C.

[0049] Thermochromic properties of the coatings based on otherchromate's are given in Table 2. TABLE 2 Thermochromic properties of thecoatings containing a chromate as a thermochromic component COMPOSITIONβ × 10⁴, ev/K 2BaO.CrO₃ 5.8 2ZnO.CrO₃ 6.2 SrCrO₄ 6.7 PbCrO₄ 5.82CaO.CrO₃ 5.8 CaCrO₄ 7.6 KAl(CrO₄)₂ 6.4 2SrO.CrO₃ 6.1 3BaCrO₄.BaSO₄ 5.8K₂CrO₄ 4.1

EXAMPLE 6

[0050] This Example illustrates the use of metal niobates, tantalates,molibdates and tungstates as thermochromic component, except tin niobateand tantalate. Lead and Niobium oxides weight of 20 g taken in thequantities corresponding to molar ratio of 1:1 were pressed into atablet and annealed at 800° C. for 100 hours. After annealing the tabletwas ground, mixed and annealed again at 800° C. for 100 hours. Afterannealing the tablet was grounded. 9.5 g of the powder of the resultantthermochromic compound was mixed with 0.4 g of potassium silicate and0.1 g of boric acid. The resultant mixture was mixed in water anddeposited onto the surface of a ceramic plate, and the plate wasannealed at 450° C. The coating had light yellow color, which becomesdark yellow at 400° C. Temperature coefficient value β=4.2×10⁻⁴.Coatings based on tin compounds were applied in the same way, but alloperations connected with preparation of thermochromic component andannealing of the coating were carried out in vacuum.

[0051] Table 3 presents temperature coefficient values of absorptionedge shift of coatings based on niobium, tantalate, molybdenum, tungstenoxides and oxides of heavy metals. All these coatings may be prepared bythe method disclosed in Example 6. TABLE 3 Thermochromic properties ofniobates, tantalates, molibdates and tungstates of heavy metals used asthermochromic component COMPOSITION β × 10⁴, ev/K SnNb₂O₆ 6.4 PbO2Ta₂O₅2.7 PbOTa₂O₅ 8.0 2PbOTa₂O₅ 6.9 2Ga₂O₃.Ta₂O₅ 4.1 TiO₂.2WO₃ 5.3ZrO₂.2Nb₂O₅ 2.1 PbWO₄ 7.0 Pb_(0.8)Mg_(0.2)WO₄ 4.6 ZnWO₄ 2.7 KGa(WO₄)₂4.8 KNbO₃ 2.8 2ZnO Nb₂O₅ 5.8 2PbONb₂O₅ 4.4 SnTa₂O₆ 7.8 BaNb₂O₆ 3.2CuNb₂O₆ 3.4 AlNbO₄ 3.0 WO₃ 7.1 BaMoO₄ 2.2 ZnMoO₄ 2.0 CaWO₄ 3.9

[0052] According to the claimed invention niobates, tantalates,tungstates and molibdates colored with transition metals ions, forinstance, cobalt, copper, chromium, nickel, manganese may also be usedas thermochromic component in the coatings. Upon heating these compoundstheir color changes on account of the absorption edge shift and onaccount of absorption band shape changes. The Examples given belowdemonstrate the process of preparing thermochromic coatings, in which asthe thermochromic component non-organic compounds are used, the coloringof these compounds relies upon transition metals ions present therein.

EXAMPLE 7

[0053] 20 g of thoroughly ground mixture of the base cobalt carbonateand tungsten oxide taken in molar ratio of 1:1 were placed into the ovenheated to 800° C. The sample was exposed at this temperature for 30hours, and thereafter the temperature was raised to 1000° C., exposureat this temperature was also 30 hours. The coating was applied with theuse of potassium silicate as the binder, the ratio of thermochromicpigment versus binder being 95:5. Reflectance spectrum of the coating isgiven in FIG. 7. It is evident from the spectrum that, upon heating thecoating containing cobalt tungstate as thermochromic component, thechange in color is connected with the absorption edge overlapping thetransparency band in the reflectance spectrum in the blue area ofspectrum and broadening of the cobalt absorption band in the green areaof the spectrum. Coating color changes from dark blue at roomtemperature to dark yellow at 400° C. Thermochromism value calculated onthe basis of color measurements to minimal and maximal temperatures isequal to TX=2.8×10⁻⁴. Color contrast of cobalt tungstate may be improvedby means of diluting the cobalt with other bivalent metals such ascalcium, magnesium, zinc. Thermochromic properties have also appropriatecompounds of nickel, copper and manganese, they may also be used forpreparation of the coatings claimed under this invention. Table 4 givesthermochromism values calculated from reflectance spectra of thecoatings which contain as the thermochromic component one of the abovementioned compounds, and in FIG. 8 as an Example, reflectance spectrumof the coating on the basis of nickel molibdate. TABLE 4 Thermochromicproperties of cobalt, nickel, manganese and copper compounds COMPOUNDColor at T = 20° C. TX × 10⁻⁴ CoNb₂O₆ blue 1.8 CoTa₂O₆ pink 1.4 CoMoO₄violet 1.2 Co_(0.8)Mg_(0.2)WO₄ blue 2.0 Co_(0.4)Mg_(0.6)WO₄ blue 1.8Co_(0.9)Pb_(0.1)WO₄ dark-blue 3.2 Co_(0.6)Zn_(0.4)WO₄ bright-blue 3.4Mn_(0.1)Zn_(0.9)MoO₄ orange 1.3 NiWO₄ yellowish-green 1.1 NiMoO₄ green1.1 NiTa₂O₆ yellowish-green 1.2 CuMoO₄ yellow 1.4 CuWO₄ brownish-yellow1.5

[0054] According to the invention more intensive color change may beattained if the thermochromic compound includes, at least, twocomponents with definite ratio of reflectance spectra. The principleconsists in the following. Let the coating have within its composition athermochromic compound characterized by gradual shift of absorption edgetowards the long-wave area of spectrum, as it is in the above Examples.Moreover, the coating contains a pigment which may have no thermochromicproperties at all, and in which the reflectance maximum resides in thesame spectral area where thermochromic component edge shift occurs (FIG.9, b) . Reflectance spectrum of the mixture in this case will be thesuperposition of reflectance spectra of two compounds (FIG. 9, c). Inthe initial position pigment reflectance band will be distinguishable onthe mixture reflectance spectrum, since at low temperature it is in thearea of thermochromic component transparency. Upon heating thethennochromic component absorption edge shifts towards the long-wavearea of spectrum and starts absorption in the pigment reflectance area.Eventually reflectance intensity in this spectral area falls tobackground level. This effect corresponds to color variation from shadeof color of the colored pigment to the color of thermochromic componentat high temperature. Naturally, thermochromism will be still higher ifthe colored pigment is also thermo-chromic, for instance, the proposedin this invention compounds of zinc and cobalt.

[0055] The Examples given below, demonstrate embodiment of the claimedcompound.

EXAMPLE 8

[0056] 15 g of bismuth oxide pre-annealed in air at 700° C., is mixedwith 1 g of commercial heat-stable blue pigment—cobalt blue (CoAl₂O₄)and 0.3 g of potassium silicate is added as the binder. The mixture isvigorously stirred, water is added and deposited onto a metallic surfaceand dried. Reflectance spectra of the resultant material are given inFIG. 9. The color of coating changes from blue at room temperature toorange at 400° C. Thermochromism rate calculated on the basis of thedata given in FIG. 10, is TX=6.0×10⁻⁴. Hence, mixing of thermochromiccomponent—bismuth oxide—with non-thermochromic pigment—cobalt blue, ledto intensification of thermochromism twice as much in comparison withthe thermochromism of pure bismuth oxide.

EXAMPLE 9

[0057] 10 g of thermochromic compound PbO.Bi₂O₃.Ta₂O₅, obtained by themethod disclosed in detail in Example 1, was mixed with 10 g ofcommercial blue-green glass C3C18 (non-thermochromic component), 0.2 gof sodium silicate is added. The resultant mixture is vigorously mixedand deposited onto a ceramic surface. The plate with coating was driedand exposed to 700° C. for 30 minutes. Coating color changes uponheating from gray-blue to gray-yellow. Thermochromism rate calculated onthe basis of temperature reliance of the reflectance spectra was 2.8,which is 25% higher than thermochromism of the initial compound.

EXAMPLE 10

[0058] 10 g of thermochromic compound Co_(0.6)Zn_(0.4)WO₄ obtained bythe method disclosed in Example 7, was mixed with 10 g of anotherthermochromic component—7Bi₂O₃.O₃ prepared as in Example 4, 0.4 g ofbinder—sodium tetraborate is added. The mixture was diluted with waterand deposited onto a ceramic surface. The coating was dried and heatedat 550° C. for 1 hour. Reflectance spectra of the coating are given inFIG. 11. Thermochromism rate of the mixture is 7.9×10^(−4,) which wasmore than twice exceeding the thermochromism of individual compounds.

1. A thermochromic material comprising a thermochromic component and a binder, wherein the thermochromic component is crystal phases based on oxides of heavy metals of I, II, III, IV, V, VI, VII, VIII groups of the Periodic System selected from the group consisting of compounds of the following general formulae: (i) (Bi₂O₃)_(1-z)(M_(x)O_(y))_(z) where z=0-0.5, wherein M is selected from the group consisting of heavy, alkali, alkaline earth metals and mixtures thereof; (ii) (M_(x)O_(y))_(m)(Bi₂O₃)_(n)Nb(Ta)₂O₅, where m=0-1, n=1-2, wherein M is selected from the group consisting of heavy, alkali, alkaline earth metals and mixtures thereof; (iii) (M_(x)O_(y))_(m)(Bi₂O₃)_(n)Mo(W)O₃ where m=0-1, n=0-12, wherein M is selected from the group consisting of alkali, alkaline earth, heavy metals and mixtures thereof; (iv) (M_(x)O_(y))_(m)(Me_(x)O_(y))_(n)Mo(W,Cr)O₃ where m=0-1, n=0-1, wherein M is selected from the group consisting of alkali, alkaline earth, heavy metals and mixtures thereof, and Me is a heavy metal; (v) (M_(x)O_(y))_(m)(Me_(x)O_(y))_(n)Nb(Ta)₂O₅, where m=0-1, n=0-1, wherein M is selected from the group consisting of alkali, alkaline earth, heavy metals and mixtures thereof, and Me is selected from the group consisting of Cu(II), Mn(II), Mn(III), Co(II), Ni(II) and Cr(III), the ratio in terms of weight percentage of thermochromic component versus binder being from 2:98 to 98:2.
 2. A material according to claim 1, wherein the thermochromic component additionally comprises a thermostable component which is non-thermochromic or thermochromic pigment, of which the area of maximum diffusion reflection lies in the same or is close to the spectral area where the temperature related change of diffusion reflection spectrum of the basic thermochromic component lies.
 3. A material according to claim 2, wherein the ratio of thermochromic component to heat stable pigment is within the range of from 50:1 to 1:30.
 4. A material according to any one of claims 1-3, wherein the binder is selected from the group consisting of silicates, borates, phosphates of alkali or alkaline earth metals and mixtures thereof.
 5. Use of the compounds of the general formula (Bi₂O₃)_(1-z)(M_(x)O_(y))_(z) where z=0-0.5, wherein M is selected from the group consisting of heavy, alkali, alkaline earth metals and mixtures thereof as thermochromic component.
 6. Use of the compounds of general formula (M_(x)O_(y))_(m)(Bi₂O₃)_(n)Nb(Ta)₂O₅, with m=0-1, n=1-2, wherein M is selected from the group consisting of heavy, alkali, alkaline earth metals and mixtures thereof as the thermochromic component.
 7. Use of the compounds of general formula (M_(x)O_(y))_(m)(Me_(x)O_(y))_(n)Mo(W,Cr)O₃ where m=0-1, n=0-1, wherein M is selected from the group consisting of alkali, alkaline earth, heavy metals and mixtures thereof, Me is a heavy metal as thermochromic component.
 8. Use of the compounds of general formula (M_(x)O_(y))_(m)(Bi₂O₃)_(n)MO(W)O₃ where m=0-1, n=0-12, wherein M is selected from the group consisting of alkali, alkaline earth, heavy metals and mixtures thereof.
 9. Use of the compounds of general formula (M_(x)O_(y))_(m)(Me_(x)O_(y))_(n)Nb(Ta)₂O₅, where m=0-1, n=0-1, wherein M is selected from the group consisting of alkali, alkaline earth, heavy metals and mixtures thereof. 